The large presynaptic nerve terminal at the squid giant synapse is unique as an experimental preparation in that it is possible to determine the physiological events involved in the release of neurotransmitters. It has not as yet been possible to carry out similar studies on vertebrate synapses due to the lack of a suitable experimental model. We have explored the possibility of using the giant calyx-type synapse in the chick ciliary ganglion as such an experimental model. We have previously found that this synapse is virtually inaccessible to direct experimentation in situ. In culture, however, the ciliary ganglion cells form calyx synapses on each other and we are investigating the electrophysiological and morphological properties of these synapses.